Process for liquefying oxides of nitrogen



Patented Dec. 31 1929 UNITED STATES PATENT OFFICE.

GHBIBTOPH BECK, O! LUDWIGSEA FEN-ON-T ASSIGNOBS '10 HANNHEII, GERMANY,

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I. G. FABBENINDUSTRIE AK'I'IENGESELL- rnoonss ron mounrxme oxrons or m'rnoenn Io Drawing. Application marl August 11,

For many industrial processes, includin for example the roduction of concentrate nitric acid from liquid nitrogen oxids, water and oxygen, a convenientdirect method of liquefying the oxids of nitrogen without previous absorption by liquid or, solid absorben'ts, is of the utmost importance. As the technical mixtures of oxids of nitrogen are often obtained highly diluted with other gases, the application of elevated pressure is for the most part inevitable, in order to obtain the vapor tension required for liquefaction, if it be desired to avoid having to deposit the nitrogen oxidsin the solidform by an expensive process of cooling to'extremely low temperatures, followed by thawing out into the liquid condition again.

' We have now found that the oxids of nitrogen can be readily'liquefied without difficulty, if the liquefaction be carried out in the presence of nitric acid. By the said addition of nitric acid the vapor tension of the liquid oxids of nitrogen consisting mainly of nitro gen tetroxid, together with a small proportion of nitrogen trioxid, to be liquefied is lowered. Consequently with even a small addition of nitric acid, far less cooling, or application of pressure, is needed than in the absence of such addition, in order to recover a given amount of liquid nitrogen oxids from a gaseous mixture with a given content of oxids of nitrogen, or in the case of undiluted nitrogen oxids'the liquefaction may be effected by cooling alone. Small amounts of nitric acid for instance about 4 per cent calculated on the amount of nitgrogen oxids to be separated are suificient for the said purpose.

The said process may be carri d out by adding nitric acid in proportion as the nitrogen oxids separate out from the mixture. Nitric acid may however also be provided, for example, in the condensing vessel, and this acid, after saturationwith nitrogen oxids, gradually becomes completely dissolved in the layer of liquid nitrogen oxids first formed, so that, finally the condensed nitrogen oxids assume the condition of a homogeneous liquid containing a little nitric acid in solution. The nitric acid to be so added is referably saturated with nitrogen oxids be orehand, as for 1928, Serial No. 299,102, and lnGermany August 18, 1927.

example by. employing it for the absor tion of the residual nitric oxid gases which ave esca ed condensation. v

T e nitric acid lowering the vapor tension may also be introduced, in the form of vapor, into the gaseous mixture prior to cooling to low temperatures so that it condenses, with the desired concentration, the nitrogen oxids, and finally, such a de gree of humidity and oxygen content may be imparted to the gaseous mixture of nitrogen oxids, which is to be cooled, that the desired amount of nitric acid is formed during the process of condensation itself. According to the purposev for which the liquefied nitrogen oxids are re uired, the concentration of the nitric acid t erein' may be arranged to be high or low, but in order to obtain the greatest possible reduction of the vapor tension, a high concentration of nitric acid a proaching the saturation point is of specidl tage.

The following example will further illustrate the nature of the said invention which however is. not limited thereto.

Ew'ample A dried gaseous mixture obtained by the combustion of ammonia and containing 190 milligrams of nitrogen oxids per liter, 'is compressed to 5 atmospheres and is passed through a cooling apparatus maintained at a temperature of -10 C. If 4 per cent of nitric acid be added to the liquefying nitrogen oxids, 5 1 per cent of the nitrogen oxids in the gaseous mixture will be recovered in the form of liquid nitrogen tetroxid, together with small amounts of nitrogen trioxid whereas without the addition of nitric acid, only 26 per cent of the'nitrogen oxids will separate out.

What we claim is: Y v

1. The process of liquefying nitrogen oxids which comprises cooling nitrogen oxids in the presence of small amounts of nitric acid.

2. The process of liqluefying nitrogen oxids which comprises coo gases containing nitrogen oxids in the presence of small amounts of nitric acid.

3. The process of liquefying nitrogenoxids ing and compressing at the same time as advan- 

